1. Field of the Invention
The present invention generally relates to compositions for use as thickening agents in aqueous systems and to aqueous well servicing fluids prepared therefrom. More specifically, the present invention relates to liquid, polymer-containing compositions for use as thickening agents to viscosify brines to provide thickened aqueous well drilling and treating fluids.
2. Description of the Background
The use of polymers in fluids, and particularly in brines, used in well drilling and treating fluids, to improve viscosity, solids removal and/or filtration control is well known. Hydroxyethyl cellulose (HEC) has typically been the preferred hydrophilic, polymeric material chosen to provide the desired thickening of brines in the oil and gas drilling industry. Compositions including HEC have long been used to viscosify drilling fluids, workover fluids, completion fluids, packer fluids, well treating fluids, subterranean formation treating fluids, spacer fluids, hole abandonment fluids and other aqueous fluids in which an increase in viscosity is desired.
Hydroxyethyl cellulose, however, does not come without its own problems. Attempts to directly incorporate HEC into a well servicing fluid as a dry powder result in the formation of fisheyes, i.e., unhydrated lumps of polymer which can result in operational problems. Potential problems include the blinding of shaker screens and the plugging of the formation.
Hydroxyethyl cellulose is not readily hydrated, solvated or dispersed in aqueous systems without the use of elevated temperatures and/or mixing under high shear for extended periods of time. In many cases, and particularly in workover operations, the equipment available for preparing the well servicing fluids does not readily lend itself to high temperature, high shear mixing. HEC polymers are particularly poorly hydrated, solvated or dispersed in aqueous solutions containing one or more water-soluble salts of multivalent cations such as heavy brines which are commonly used in well servicing fluids.
One successful attempt to solve these problems was disclosed in U.S. Pat. No. 4,330,414, incorporated herein by reference. The '414 patent disclosed a water miscible, polar, organic liquid for use as a solvating agent to form a semi-solid to viscous mixture with the hydroxyethyl cellulose.
In another approach to solving these problems, the polymer was added in the form of a solution, colloid or other suspension dispersed in a non-solvent carrier medium, e.g., an oil-based liquid such as diesel oil or kerosene. For example, see U.S. Pat. No. 4,622,153, incorporated herein by reference. However, it was found that compositions prepared in accord with the disclosure of the '153 patent experienced undesirable settling and hard packing of the HEC when stored under static conditions for extended periods of time. Resuspension and dispersion of the HEC was a time consuming process and required special equipment. Accordingly, solutions prepared in accord with the disclosure of the '153 patent were not conducive to use at on-site drilling, workover or completion operations.
Another effort to solve these problems was disclosed in U.S. Pat. No. 4,615,740, incorporated herein by reference. The '740 patent disclosed a liquid, polymer-containing composition for viscosifying oilfield brines including, in addition to hydroxyethyl cellulose, an oil-based liquid, an aluminum phosphate compound and optionally a surfactant.
While the thickening compositions described in the foregoing patents have been successful, changing times have created a need for new and improved thickening compositions. Environmental concerns, and particularly toxicity concerns, have arisen with respect to well servicing fluids including oil-based components such as those described above. As the number of offshore drilling operations has proliferated and as environmental concerns have increased, the continued use of these prior compositions has become unacceptable. Recent changes to the NPDES Offshore Discharge Permit requirements prohibit the discharge of fluids containing oil into offshore environments. The changes essentially prohibit the use of oil-containing products in offshore environments where discharge may occur. Accordingly, oil-free completion and workover fluids, including oil-free thickening agents, must be used in operations in environmentally sensitive areas covered by these regulations.
Therefore, there has developed a pressing need in the well servicing industry to develop and implement the use of safer and less toxic well servicing fluids. The present invention solves that need.